Dragon, Deep Great Wyrm (CR 25)

Gargantuan Dragon (Earth)
Alignment: Always Chaotic evil
Initiative: +0; Senses: blindsense 60 ft., darkvision120 ft., low-light vision, and keen senses
Languages: Draconic

AC: 44 (-4 size, +38 natural), touch 6, flat-footed 44
Hit Dice: 49d12+312 (565 hp); DR: 20/magic
Fort +30, Ref +22, Will +28
Speed: 40 ft., fly 200 ft. (clumsy) , burrow 20 ft., swim 30 ft.
Space: 20 ft./15 ft. (20 ft. with bite)
Base Attack +39; Grapple +63
Attack: 1 Bite +47, 2 claws +42, 2 wings +42, 1 Tail Slap +42, 1 crush +42, 1 Tail sweep +42 melee; Breath +47 ranged
Damage: 1 bite 4d6+13, 2 claws 2d8+6, 2 wings 2d6+6, 1 tail slap 2d8+19, 1 crush 4d6+19, 1 tail sweep 2d6+19, Breath weapon 24d8 (37)
Special Attacks/Actions: Breath Weapon, fear (DC 35), SR 34
Abilities: Str 35, Dex 10, Con 27, Int 24, Wis 23, Cha 22
Special Qualities: True seeing, detect magic, freedom of movement, transmute rock to mud, passwall, stone shape, charm immunity, cold and fire resistance 10, Snake form 4/day, humanoid form 4/day. CL 17. Can also cast cleric spells and those from the Earth, Evil and Trickery domains as arcane spells.
Feats: #Feats: 17
Class Skills: Hide, Move Silently, Skill points: 349, and Swim
Advancement: 40+ HD (Gargantuan)
Climate/Terrain: Underground
Organization: Solitary or clutch (2-5)
Treasure/Possessions: Double Standard

Source: Monster Compendium: Monsters of Faerûn

Breath Weapon (Su): A deep dragon's breath weapon is a cone of flesh-corrosive gas 24d8 damage, reflex save for half (DC 37). Using a breath weapon is a standard action. Once a dragon breathes, it can't breathe again until 1d4 rounds later. A blast from a breath weapon always starts at the dragon's mouth and extends in a direction of the dragon's choice.

True Seeing (Su): Deep dragons have a permanent form of true seeing (as the spell).

Snake Form (Su): A deep dragon can assume its serpentine (legless) form four times per day. This form is the same size as the dragon, but able to move through smaller tunnels because it has no legs. The dragon's AC worsens by -6 in this form. It moves at 30 feet, flies at 10 feet (poor), and swims at 30 feet. It loses all claw attacks in this form, but can constrict opponents, dealing 3d8 damage.

If the dragon hits with this attack, it can make a free grapple check (without provoking an attack of opportunity). If it wins this check, it grabs and constricts its victim, automatically dealing the above damage every round until the opponent dies or breaks free.

Humanoid Form (Su): A deep dragon can polymorph self into a humanoid form four times per day. It can alter the size, shape, hue, and features of its bipedal form to resemble a human, elf (surface or drow), half-elf, half-orc, orc, hobgoblin, dwarf, duergar, or any similar creature of Medium-size. If the dragon wears armor to improve its AC in this form, the armor is destroyed when the dragon changes shape.

Earth Subtype

This subtype usually is used for elementals and outsiders with a connection to the Elemental Plane of Earth. Earth creatures usually have burrow speeds, and most earth creatures can burrow through solid rock.